In conventional filament winding operations, a strand of fibrous material such as glass or carbon, is continuously passed through a bath to impregnate it with liquid resin. The fully impregnated strand can then be filament wound or otherwise fabricated into an article such as a pipe or a tube. After a curing reaction, the resin solidifies and forms an integral composite structure with the fibers which contains little or no entrained air to interfere with load transfer between these components.
The fibrous strand used in such winding operations consists of multiple substrands or filaments. For complete impregnation, it is essential that all of the individual filaments are fully coated with resin during passage through the path. If filaments are incompletely coated, air spaces or voids result during filament winding which cause significant loss of mechanical properties. Total impregnation is difficult because there are normally from 50 to 100 filaments per strand which are tightly intertwined together, and the viscous resin cannot readily penetrate this mass.
The degree of fiber impregnation is related to the rate of resin absorption into the fibrous bundle. Resin is absorbed by capillary action as it wets the filament surfaces and displaces the air inside the bundle. Above some critical speed, impregnation decreases as fiber speed through the bath increases. The resin does not have time to be absorbed into the bundle. The vastly dissimilar polarities of the fibers and resins cause poor wetting (surface absorption) of the liquid on the filaments which then reduces the capillary pressure drawing resin into the fiber bundle. The rate of impregnation is therefore relatively slow, and longer contact times are required to saturate the bundle. Certain fibers, such as carbon, are wetted even less than glass by the resin. These strands have lower capillary pressures and require even more contact time for complete resin impregnation of the bundle. Voids are much more prevalent in these cases than for glass strands.
The most frequently used method of improving the impregnation degree is with chemical binders. These complex molecules contain two different chemical polarities. One polarity is attracted to the inorganic surfaces and is easily absorbed onto the strands. The other has a high affinity for the organic liquids and is easily dissolved by the resin. The binder is coated onto the filaments in a separate step during strand manufacture and dried in place to insure it is not physically rubber off during fiber handling. When this strand is passed through a resin bath, the resin easily dissolves the organic end of the binder, allowing the resin to absorb onto the strand surface more completely than for uncoated fibers. Capillary pressure inside the bundle is increased, forcing resin into the bundle at higher rates. For similar contact times of resin and fibers, higher degrees of impregnation then result.
Another advantage of chemical binders is their ability to increase the stability of the resin-fiber bond after curing. Poor fiber wetting causes a low cohesive energy of the cured resin to the fibers which reduces bond stability. More polar molecules, such as moisture, then easily displace the solid resin from the strand by hydrolysis. Chemical binders, because of their different polarities matched to both the resin and fibers, hold these components together with higher cohesive energies. Thus, bond disruption by hydrolysis and heat is effectively retarded.
Ultrasonics has been used to increase the rate of chemical interactions of various materials. For example, it allows soldering of dissimilar metals by driving molecules from one surface into the other and providing efficient mixing and joining of the combination. Ultrasonics greatly increases the rate of electroplating operations by increasing the interaction of the liquid plating solution with the solid electrode or workpiece to be coated. Ultrasonics has also been used to increase the interaction of glass and resin. In that application, bare fibers are subjected to ultrasonic energy as they pass through a resin bath to eliminate air entrapped in the bundle. Ultrasonics increases capillary action by effectively mixing entrapped air from the fiber bundle with the liquid resin, thus improving impregnation degree. This procedure is used only where binders are not effective or practical. Ultrasonics is not used in conventional filament winding operations because bond permanence is not improved, as with binders. Further, binders permit maximum resin impregnation or saturation of the fibers without the expensive ultrasonic energy.
With improvements in process technology, faster winding speeds have become possible to increase production rates. While binders provide satisfactory impregnation at conventional fiber speeds, under 200 feet per minute, at higher speeds reduced resin absorption begins to occur with consequent loss of mechanical properties. At such high speeds, contact time of the resin and strand is reduced to less than 0.35 seconds which does not allow sufficient time for the dried binder to be redissolved by the resin as is necessary before it can improve the wetability of the strand. Such behavior is analogous to chemical detergents (molecules with similar bipolar structures to binders) which must be dissolved in water before they can emulsify oils or dirt. Thus, production efficiency of filament winding operations is now limited by the relatively slow impregnation rate of the fiber bundles even when they are coated with binders.